Silicon composites have great potential as an anode material in lithium-ion batteries. On account of the large volume change of the silicon during repeated charging/discharging these silicon composites cannot be used as an anode material.
Intensive efforts have therefore been made to improve cycle stability through use of composites of silicon-graphite, graphene-nanosilicon, silicon-carbon nanotubes, silicon-carbon nanowires, carbon encapsulated with silicon and silicon encapsulated with carbon. Methods for producing these composites are for example pyrolysis, grinding or CVD processes. (Zhang et al., Nanoscale, 5 (2013) 5384 and Kasavajjula et al., Journal Power Sources 163 (2007) 1003).
Magasinki et al., Nat. Mater. 9 (2010) 353, describe the production of a silicon-carbon composite starting from monosilane and propene in a two-stage CVD process. In a first step silicon is applied atop a carrier by introducing an SiH4/He mixture into a tubular reactor at 700° C. under vacuum. Subsequently, carbon is applied atop this silicon by introducing propene into the tubular reactor under the abovementioned conditions.
WO2011/006698 discloses a process for producing a nanostructured silicon-carbon composite in which a submicron silicon powder is added to a carbon-containing mixture produced by reaction of a hydroxyaromatic compound with an aldehyde and the mixture is carbonized at 500° C. to 1200° C.
A further variant is, according to Wang et al., Electrochem. Commun. 6 (2004), 689, the addition of nanocrystalline silicon powder to a gelling resorcinol/formaldehyde mixture which cures at 85° C. for 10 hours. This mixture is a compact block which is converted at 650° C. into a silicon-carbon composite comprising 40% carbon.
EP-A-2782167 discloses a process for producing an Si/C composite in which silicon and lignin are reacted in an inert gas atmosphere at at least 400° C.
US2009029256 discloses a process for producing an Si/carbon composite in which a mixture of alkaline earth metal and the silicic acid/carbon composite is heated in an inert atmosphere.
Laid-open specification DE 102009033251 A1 proposes a three-stage process for producing a composite from silicon-carbon or silicon-tin. Si or Sn nanoparticles are generated and subsequently introduced into an organic polymer. In the third step the system composed of polymeric matrix and the nanoparticles is pyrolyzed.
Patent application DE 102016203324.7 discloses a process for producing a silicon-carbon composite powder in which a silane is reacted in a hydrocarbon-containing gas stream in a hot wall reactor.
Conventional processes often comprise more than two stages and are therefore complex or they have the disadvantage of affording only small laboratory-scale quantities. The latter is for example caused by undecomposed precursors being deposited in the interior of the apparatus and requiring recurrent removal with shutdown of the apparatus. Accordingly, such processes often cannot be kept running stably over prolonged periods.
The present invention had for its object to provide a process which allows production of a composite based on silicon and carbon in fewer steps and using input materials available on a large industrial scale.